The time difference between an upstream and a downstream ultra sonic signal is proportional to the flow, and used in time of flight ultra sonic meters as a measure of the flow. If the time difference Δt becomes longer than the duration of the period of the ultra sonic signal, an exact detection of the time difference becomes difficult due to the signal periodicity. In order to avoid this problem, known solutions provide detection circuits that are practically independent of the extent of Δt, i.e. the detection circuit makes measurement possible on ultra sonic flow meters, which have a Δt longer than the period of the signal. An example of such a prior art detection method and circuit—also called a trigger—is described in the following, where the envelope of the upstream and downstream signal play a significant role.
The basic purpose of a trigger in a transmit time ultrasonic flow meter is to “point” out the time of arrival of the ultrasonic signal. This is used to measure both the difference between the upstream and downstream transmission time and to measure the two transmission times. From these values the flow Q can be calculated according to (1):
                    Q        =                  k          ·                                    Δ              ⁢                                                          ⁢              t                                                      t                1                            ·                              t                2                                                                        (        1        )            where Δt is the difference time, t1 and t2 the transmission times and “k” is a constant dependent on the geometry of the tube. If the media is known, the measurement of the two transmission times can be replaced by measuring the media temperature and calculating the sound speed C from knowledge of the variation of the speed of sound with temperature:Q=k·Δt·C2  (2)where Q is the flow, k is a constant, Δt the time difference and C the sound speed.
FIG. 1 is an illustration of the receive signals—the first arriving is the result of the sound pulse travelling in the flow direction, 1, and the second is the result of the sound pulse travelling against the flow direction, 2. In the following the term zero crossing will be used, in practical implementations this will be signal zero (the middle of the range of voltage used in the implementation) or some value either a little over or under the signal zero. Still referring to FIG. 1, the basic problem is to trig or initiate the time measuring circuit with the “same” zero crossing in the upstream and the downstream sound pulse, otherwise a wrong Δt is measured. P1 and P2 are to be imagined as same zero crossings because each have a distance of 3½ periods from reception of the sound pulse. Also indicated in the Figure is the period tsig of the sound signal and the time difference Δt.
FIG. 2 shows how a prior art ultra sonic flowmeter uses the envelope of the ultrasonic signal to achieve a zero crossing detection that is independent of the length of Δt. The incoming signal (S1) is rectified (B1) and the result is (S2). This signal (S2) is fed through a band pass filter with non-minimum phase behaviour (B2). Non-minimum phase systems have the transient property that their initial direction of response is in the opposite direction of the final value—as a consequence, if the filter parameters are chosen appropriately, the output of the filter (B2) will have a well defined zero crossing indicating the receive time. Furthermore this zero crossing will be independent of the amplitude of the receive signal. The signal on the output of the filter is seen as (S3). The zero crossing of the signal (S3) is detected by the zero cross detector (B3), this signal (S4) is arming the zero cross detector (B4). After arming the zero cross detector (B4), the next positive or negative, dependent on the actual implementation), zero crossing in the original receive signal (S1) is detected by (B4) resulting in the signal (S5). The time where the signal (S5) changes from low to high is measured relative to the time of the transmit burst (or relative to another time with a known relation to the transmit time). If the time between the zero crossing of S3 and the following zero crossing of S1 is very short, there is a risk of detecting two different zero crossings of S1, due to random noise. To avoid this situation it is detected if the two zero crossings are too close, and if this is the case, the transmit signal is inverted—and hence the receive signal. The consequence of the inverted receive signal is that the previously very short time difference between S3 and S1 is now close to one half period of the receive signal. One can chose to measure transit time on the signal zero crossing (S5) or on the zero crossing of the signal (S4). After having calculated a time as described above for an upstream signal, the same procedure is used on the downstream signal. From these two times, a difference time is established and the flow Q calculated.
The described detection method works well in systems were the span of Δt is unknown. This is the case for a general purpose ultra sonic flow meter as the one described above, which are used for a variety of tubes having different diameters. This type of ultra sonic flow meters must be able to cope with a very wide span of Δt. However, in some systems, the span of Δt is limited by fluid velocity and/or the mechanical arrangement of the ultra sonic transducers which means that the ultra sonic converter can be designed according to other and less demanding principles. Such a limitation in Δt is the case, if the two ultra sonic transducers mounted in the tube are very close to each other. It will then be known that Δt e.g. will have a maximum value of e.g. 1 μs. Further, a drawback of the prior art design described above is the relatively extensive and thus costly use of electronic circuitry. Another weakness of the method is the dependence on a stable signal envelope. If for instance a single pulse in the receive signal has a lower amplitude due to electrical noise or particles/air bubbles in the liquid, the envelope form changes, and consequently a wrong Δt will be calculated.
Based on the foregoing, the object of the invention is to provide a detection method which is realized in a simpler way and with fewer electronic components, and still gives a reliable statement as to the difference in transmission time, Δt.